Here was my little misadventure last month at Tradewinds. I think I have the dubious honor of being the only sailor to take an unintended swim that weekend. Fortunately, no damage and the water was warm.
http://www.youtube.com/watch?v=RExuf_WlbRs My trap hook had some kind of spring clip retainer. When I went out, the hook/clip must have been just resting on the edge of the ring. As soon as I put weight on it, splash. Needless to say the clip was removed.
My crew's first instinct with the main sheet wasn't quite spot on either... Later, trying to uncleat the center-sheeted main from in the water was a real PITA.